The Fluid Catalytic Cracking (FCC) process is now the predominant process in the petroleum refining industry for the boiling range conversion of the high-molecular weight hydrocarbon fractions of petroleum crude oils to more valuable gasoline, olefins and other products which may be passed to other refining processes such as hydrocracking.
Various types of FCC process unit (FCCU) exist with variant designs being offered by technology licensors in the industry. In principle, however, all hark back to the original design from Esso in the early 1940s with a reactor vessel and a regenerator vessel with a finely-divided solid, particulate catalyst circulating continuously between them. Current designs carry out the cracking of the feed in a riser which is a substantially vertical pipe with a feed injection zone at the bottom into which hot catalyst from the regenerator is fed to meet the incoming feed which is injected into the mix zone through nozzles with aid of steam. The regenerated catalyst enters the riser below the feed mix zone and is lifted up into the mix zone with lift gas. In the riser the vaporized feed is cracked into smaller molecules of vapor by contact and mixing with the hot catalyst; the cracking reactions take place in the catalyst riser within 10 seconds, typically 2-4 seconds. The mixture of hydrocarbon vapors and catalyst flows upward to enter the reactor vessel which now functions as a disengager to permit separation of the spent catalyst from the cracked hydrocarbon vapors. The spent catalyst flows downward through a steam stripping section to remove any hydrocarbon vapors before the spent catalyst returns to the catalyst regenerator where the coke which accumulates on the catalyst particles as a result of the carbon rejection which is the characteristic feature of the process is burned off with air to restore catalyst activity and selectivity as well as providing heat by the exothermic combustion of the coke to maintain a heat balance in the unit with the endothermic cracking reactions.
The feedstock to the FCCU is usually that portion of the crude oil that has an initial boiling point of 340° C. or higher at atmospheric pressure and an average molecular weight ranging from about 200 to 600 or higher. This portion of crude oil is typically the high boiling fraction from the vacuum distillation tower often referred to as heavy gas oil or vacuum gas oil (HVGO or VGO) although in recent years cracking of residual fractions has become more common and in addition, the end points of the gas oil fractions have increased in order to secure the maximum economic profit from available crude sources. With this trend towards higher boiling feeds, however, have come attendant difficulties. Not only do the feeds tend to produce more carbon during the process (an inevitable result of the carbon rejection) but “coking” or the formation of highly carbonaceous fouling deposits in the unit has become more prevalent and, with continued accumulation, can lead to shut down of the unit. Some units have experienced unexpected feed zone coking that forced unit shut-down for cleaning. Existing operating envelopes including factors such as feed nozzle minimum pressure drop and ratio of feed injection steam to fresh feed were found to be inadequate for predicting coke growth in the feed zone.
The formation of coke fouling deposits may occur at various locations in the unit, including the interior of the reactor as a black deposit on the surface of the cyclone barrels, reactor dome, and walls, the transfer line from the reactor vessel to the main fractionation column, in the slurry oil circulating slurry system where it is likely to plug up exchangers, resulting in lower slurry circulation rates and reduced heat removal. Another site where coking is often encountered is in the riser, notably in the feed injection zone where the stream of hot catalyst from the regenerator meets the pre-heated feed injected with steam through the injection nozzles. Coking in the riser is a particular problem since reductions in the already limited size of the riser can increase the pressure drop, leading to catalyst circulation capability problems in the upper end and loss of throughput.
Coke-induced fouling is believed to take place in areas where condensation of hydrocarbon vapors occurs. Unvaporized feed droplets readily collect to form coke precursors on any available surface. Heavier boiling components in the feed may be very close to their dew point, and they will readily condense and form coke nucleation sites on even slightly cooler surfaces. Equilibrium flash vaporization calculations often indicate that heavy material is not vaporized at the mixing zone of the riser which is exacerbated by residue processing and short riser residence times also contribute to coke deposits since there is less time for heat to transfer to feed droplets and vaporize them.
Higher boiling range, higher aromaticity feedstocks might be expected to result in worse coking rates but commercial experience has shown that feed quality alone is a poor predictor of which units will experience coking problems. While existing commercial practice has been to increase feed injection steam based on experience, this has been done solely on an basis of experience but provides no guideline based on theory and calculation.
An online article by McClung of Engelhard, “Monitoring FCCU Feed Vaporization”, available at http://www.refiningonline.com/engelhardkb/crep/TCR1_7.htm, describes an empirical approach by which feed dew point and feed vaporization could be estimated and used to reduce the extent to which unvaporized feed droplets undergo condensation to coke on unit surfaces, especially in the cool spots in the transfer line (uninsulated hangers) or in the plenum (metal surfaces cooled by wet steam). The approach proposed by McClung was to assume a riser operating temperature above the feed dew point by assuming perfect mixture like in the flash model, but in reality it is not. It would therefore be desirable to develop an improved method of predicting the inherent tendency of a heavy petroleum oil feed to generate coke deposits in the FCC riser while accounting for imperfect mixing in the feed zone. Ideally, the method should be inexpensive, readily available at the refinery, capable of producing quick results and provides ease of monitoring and use so that operating conditions in the FCCU may be adapted to the feed(s) being processed.